Despite strong interest in short term process effects on dinoflagellates there have been few investigations on the seasonality of marine bioluminescence. Long term aspects of the development of bioluminescence are unknown for most oceans. The present study was designed to cast light on this question. A station for measuring bioluminescence was established in August 1993 at San Clemente Island (SCI), 100 km offshore of Southern California. Bioluminescence was measured with a moored bathyphotometer (MOORDEX) hourly through February 1996. Other environmental parameters such as nutrients, chlorophyll, and associated plankton species were measured and collected on a monthly and quarterly basis (Lapota et al. 1997). In the present study, plankton samples were collected and tested for bioluminescence on a quarterly basis to: 1) determine which dinoflagellate species were bioluminescent and 2) observe differences in light output on a seasonal basis. The latter is an important consideration because seasonal changes in bioluminescence from dinoflagellates might possibly indicate a response to regional seasonal environmental changes. These factors include the available nutrients and light for the photosynthetic species (Ceratium, Gonyaulax1, Pyrocystis) and the availability of diatoms and smaller algal cells consumed by the heterotrophic Protoperidinium dinoflagellates. Seasonal changes in light output will affect the bioluminescence light budget of all species. Published light budgets are limited and specific for limited oceanic areas, the number of species tested, or modeled to predict bioluminescence output based on the calculated cell surface area. This study will complement earlier laboratory work and enlarge these observations by identifying distinct seasonal differences in bioluminescence of open ocean dinoflagellates over a two year period.